OBJECTIVE: This study investigated the influence of impaired cerebellar locomotion function on the magnitude and structure of stride-to-stride fluctuations in the walking pattern. On the basis of studies reporting a dependency of variability magnitude and structure on the walking speed, we hypothesized that patients with cerebellar ataxia (CA) would show alterations of gait variability in a speed-dependent manner. METHODS: 11 patients with CA [7 idiopathic sporadic ataxia, 4 inherited spinocerebellar ataxia] and 11 healthy subjects (HS) walked on a treadmill for 5-min periods at their preferred walking speed and at 20%, 40%, 70%, and 80% of maximal walking speed. The variability magnitude of stride time, stride length and base width was calculated, and long-range correlations were detected by a detrended fluctuation analysis. RESULTS: Both temporal and spatial gait variability were impaired in CA. Variability magnitude and structure of all examined parameters depended on the walking speed. The preferred walking speed of patients was linked to minimal levels of stride time and stride length variability magnitude and to the strongest correlations within the fluctuations of these parameters. Long-range correlations were present for all examined gait parameters in patients and HS. Compared to HS, patients showed alterations in the speed dependency of stride time and stride length variability, with increased variability occurring at slow and maximal pace, whereas base width variability remained unaffected. CONCLUSIONS: Gait variability and therefore walking stability is critically dependent on the walking speed in patients with CA. At preferred walking speed, however, variability is minimal and similar to HS.